The invention pertains to energy generation. Particularly, it pertains to the generation of energy in small amounts by small devices, and more particularly to microcombustion energy generation.
Batteries have served well as small, portable electric power sources. But they require a relatively long time to recharge or if not recharged, contribute to an increasingly objectionable waste disposal problem. Furthermore they suffer from a low volumetric or mass energy density (compared to that of liquid fuels). Fuel cells may some day overcome the above issues, but presently are either very sensitive to fuel impurities (such as CO in polymer-based fuel cells operating on H2) or require very high operating temperatures, which delay startups and cause shortened service life due to thermal cycling stresses.
The proposed microcombustion engine (MCE) and/or microcombustion generator (MCG) operates three times as long between recharges (requiring less than 1 minute) as a battery of similar volume (e.g., as large as a butane xe2x80x9cBicxe2x80x9d lighter), and does not pose a disposal problem when it needs to be replaced. Alternatively it provides fifteen times more heating energy, or output mechanical work when preferred, than a comparable battery.
The present invention, in part, concerns electrical control of piston synchronization for a microengine having at least two free pistons (pistons with no mechanical linkages). The dimensions of the microengine are typically one millimeter (mm) or less, which is less than the quenching length for combustion in typical fuels. Thus, it is difficult, if not impossible, to initiate combustion with conventional spark plugs. To overcome this difficulty, the microengine operates in a knock mode (i.e., homogeneous auto ignition), where the fuel is compressed to a pressure and temperature high enough to initiate combustion without a spark. In a two-piston microengine, combustion occurs on each cycle where the two pistons meet. Preferably, this is near the center of the engine cylinder, where fuel can be provided and exhaust disposed of efficiently. This requires the motion of the two pistons to be synchronized. If the pistons are not synchronized, the point of combustion will occur away from the center of the microengine, causing the microengine to operate less efficiently, or perhaps cease to operate at all. This invention utilizes electrical methods to synchronize the pistons. In a conventional engine, the pistons are synchronized by mechanical linkages. In a free-piston engine, this is not possible. If the pistons are used to generate electrical power, then the means for generating electrical power can also be used to sense the synchronization error and to apply force to the pistons to correct the synchronization error. Electromagnets are used to sense the positions of the pistons and apply forces to the pistons, in addition to generating electrical power. However, many of the external control circuits are applicable when other types of mechanical to electrical transducers are used, such as piezoelectric or electrostatic transducers.
The basic concept of the proposed engine/generator is to take advantage of the high energy density of available hydrocarbon fuels, which range from 42-53 MJ/kg (11.7-14.7 kWh/kg or 18,000-22,000 Btu/lb.). But rather than be dependent on the proper operation of active/catalytic surfaces in fuel cells, the work potential of combustion engines is harnessed for the conversion from chemical to electrical energy. The main challenge for small, portable systems is to have very small functioning engines that efficiently achieve outputs of ten watts or less.
The features of the present MEMS (i.e., micro electromechanical systems) engine are as follows. It is a linear-free piston engine with complete inertial compensation. The engine is without piston rings, without intake or exhaust valves, and without a carburetor. The engine utilizes xe2x80x9cknockingxe2x80x9d combustion to overcome wall quenching in combustion chambers smaller than the classical quenching distance. It implements high adiabatic compression ratios within small cylinder and piston geometries.
This engine""s features come from three areas. One is the combining an opposed dual piston engine design with the advantageous exhaust gas and fresh gas mixture charge scavenging and inherent inertial compensation. Another is a free piston engine design having gas springs. It uses xe2x80x9cknockingxe2x80x9d rather than diesel or spark-ignition and an embedded magnet-in-piston, in an engine-generator configuration. The piston size is (square or round cross section) of 0.1-3 mm, and length of 5-14 mm. This system may be fabricated in ceramic or silicon via deep reactive ion etching (DRIE) or other process within a tolerance band of xc2x12.5 xcexcm. The top and bottom layers may be composed of sapphire, Pyrex, silicon or other accommodating material. Silicon carbide and metal may also be used in the structure of the engine.
The dual-opposed, free-piston microcombustion engine (MCE) generator has advantages over existing power sources. In contrast to fuel cells, no catalytic films are poisoned by trace constituents such as SO2 or CO, as is the case with (low- and high-temperature) polymer and ZrO2-based fuel cells, whose service life is shortened by thermal cycling; no high-temperatures need to be achieved with the MCE before operation can begin, as with ZrO2 fuel cells. At the same time, the MCE with its assumed 20 percent conversion efficiency is likely to be less efficient than a fuel cell.
The energy density of batteries (xe2x89xa61 MJ/kg) is less than ten percent of the 40-50 MJ/kg of hydrocarbon fuels; a xe2x80x9cBicxe2x80x9d lighter storing the same volume of liquid butane as a xe2x80x9cCxe2x80x9d size battery (18 cm3, allowing for a 1 mm-thick container wall) packs 0.58 MJ of combustion energy or 0.12 MJ electrical energy at a conservative twenty percent engine conversion efficiency. This is compared to the 0.039 MJ in a battery for 7.8 Ah at 1.4 V. The present MCG is also easier and quicker to xe2x80x9crechargexe2x80x9d in the field by simply refilling the fuel, whereas a battery needs an electrical outlet and time to recharge.
At the same time, the design of this combustion engine was dictated by several considerations. Engines with a crankshaft would either self-destruct within a short time under xe2x80x9cknockingxe2x80x9d combustion or would not achieve compression-ignition when reduced to MEMS sizes (a piston diameter on the order of 1 mm), and therefore could not be scaled down to such sizes. Related art engines suffer from a much larger piston-to-cylinder sidewall friction, wear (shorter service life) and thus efficiency losses. And by operating under a fixed compression geometry, they are much less flexible in terms of the required fuel properties than free-piston engines.
Knocking occurs when a highly compressed air-fuel mixture in the combustion chamber is compressed rapidly and sufficiently. By compressing the mixture sufficiently fast, heat from this adiabatic event is added to the mixture. The heat from the compression will raise the temperature of the air-fuel mixture enough to ignite itself.
Engines with individual piston chambers cannot as effectively flush out exhaust gases and charge a fresh combustible mixture because their exhaust and intake ports have to be attached to one piston cylinder, rather than situated between two opposed pistons sharing a common combustion chamber.